Gene transfer into neural cells has grown into a big field in neuroscience. The usage of gene transfer is ranging from treatment of genetic diseases, tumours and acquired degenerative encephalopaties such as Alzheimer's disease and Parkinson's disease to being a powerful tool in the study of biological mechanisms. An important application of gene transfer is gene therapy, which is when a therapeutic gene is inserted into the cells by ex vivo or in vivo techniques. One of the obstacles to overcome with gene therapy is to get the gene into the right cell type. The choice of cell depends on the nature of the disease. One example is cystic fibrosis, where clinical trials have already started to deliver a vector with a correct copy of the damaged gene (the CFTR gene) as an aerosol into the lungs. But even if the disease is manifest in the lung it is not certain that the correcting gene will enter the right cell type. In other diseases such as Haemophilia B where a blood clotting factor (IX) is missing in plasma, it is not as important to reach the damaged cell type. Even if the liver normally makes the clotting factor, it does not matter if the therapeutic gene is inserted in muscle cells, fibroblasts or even blood cells as long as the clotting protein is produced in therapeutic amounts and with the correct post-translational modifications. The protein accessibility to its target and the immunological status is also important. A protein that is normally expressed only inside the blood-brain barrier could for example be immunogenic if exposed on the outside.
There are two different approaches to deliver the DNA (target genes) into the cells. The first is the usage of non-viral vectors to insert the DNA. The non-viral approach consists of methods like direct injection of the DNA, mixing the DNA with polylysine or cationic lipids that allow the DNA to be internalised. Most of these approaches have a low efficiency of delivery and transient expression of the gene. The second and more widely used approach to insert the DNA is by using viral vectors. Viruses have evolved a mechanism to insert their DNA into cells very effectively, but the side effect is that humans have evolved an effective immune response to eliminate viruses from the body.
To function as a viral vector in the nervous system, the vector should have certain properties. Since almost all cells in the brain are non-dividing, the vector must be able to infect non-dividing cells. A good vector also needs to be non-toxic to the cells in the dose required for infection (direct cytotoxicity i.e. by capsid proteins and antigenicity). It should be replication deficient to prevent the virus from uncontrolled spreading and damage to the cells. After the DNA has entered the nucleus it should integrate in a site-specific location in the host chromosome or become a stable extra-chromosomal element (episome). The desired gene should be expressed without interfering with the cellular expression machinery.
Viral vectors used for gene delivery into the nervous system are herpes simplex-1 virus, adenovirus, adeno-associated virus and retrovirus such as lentivirus. All the viruses pathogenicity genes have been deleted and their ability:to replicate has-been incapacitated.
Adenoviruses are good candidates for gene therapy towards the nervous system for a number of reasons. They can infect both dividing and non-dividing cells, the viral genome is relatively stable and is easy to manipulate by recombinant DNA techniques. Replication of the virus is efficient in permissive cells and the pathogenicity is low. One of the obstacles to overcome with all viral vectors is to achieve a sustained expression. The viral vector evokes an immune response that is both cell mediated and humoral and the infected cells may become destroyed by the inflammatory response within a couple of weeks. The immune response evoked by adenoviral gene transfer is however different in the brain compared to in the peripheral tissues. The immune response in the brain is not sufficiently strong to eradicate the adenovirus infected cells. However, if the individual has had a previous exposure to the adenovirus or was inoculated with the virus later, a strong inflammatory response can be evoked also in the brain. Consequently, adenovirus serotypes of low prevalence in the society should preferentially be used as vectors to be more successful as gene delivery vectors.
The adenoviruses are a family of DNA viruses that can infect both dividing and non-dividing cells. They do not usually integrate into the host chromosome, instead they are replicated as extra-chromosomal elements inside the nucleus of the host cell. Adenoviruses can bind to a range of different cell types. The clinical picture of an adenovirus infection is often respiratory infection or gastro-enteritis. A tonsillitis similar to a streptococcus A infection is also not uncommon. Some of the adenovirus serotypes can cause epidemic keratoconjunctivitis or in some cases even meningitis or encephalitis.
There are at the present 51 known serotypes of adenovirus, which have been divided into six different subgenera, A-F, depending on their biological properties and genetic homology. Virus within the same subgenus shares more than 50% DNA homology whereas viruses in different subgenera have less than 20% homology (Wadell G., Adenoviruses (adenoviridae): General features. Encyclopaedia of Virology. Ed. Webster R. G., Granoff A., Academic Press Ltd London, pp 1-7, 1999).
Human adenoviruses are non-enveloped and about 80 nm in diameter with a 36 Kbp double stranded DNA. The virion capsid is composed of 240 hexon capsomers and 12 vertex capsomers. An antenna-like fibre projects from each vertex capsomer (located at the corners of the icosahedral capsid). The epitopes capable of making serotype specific antibodies and hence also the epitopes determining the serotype are located on the external portions of the hexons and on the most distal knob of the fibre.
Infection starts with the attachment of the fibre knob to a cellular receptor on the permissive cell. The cellular receptor for the virus fibre is coxsackievirus-adenovirus receptor (CAR) for all subgenera except subgenus B. Additional cellular receptors for adenoviruses are the major histocompatibility complex class I (MHC-I) alpha2 and sialic acid. The viral penton base then binds to the cellular integrin αvβ3 and the virus is internalised by endocytosis into an endosome. Upon fusion with a lysosome, the pH is lowered leading to alterations in the viral capsid, releasing the virion from the endo-lysosome. The virion is then transported to the nucleus where the replication and transcription takes place. The spliced mRNA is translated in the cytoplasm. Production of the fibre protein can be detected 9-11 h after infection. The structural proteins are then translocated into the nucleus where assembly of new virions takes place.